As shown in FIG. 1, a prior art water heater 10 comprises a plurality of heat collecting pieces 12 arranged orderly in a row and a hollow metal body 14 provided respectively at both sides thereof with an opening 16 and with a V-shaped body 18. During the heating operation of the water heater 10, the exhaust fume so generated is let out in the directions indicated by the arrows. However, the exhaust fume is not let out directly through the exhaust holes 19 before the heat of the exhaust fume is absorbed by the heat collecting pieces 12. The heat so absorbed is then transmitted to heat the water in the water pipes which pass through the heat collecting pieces 12, which are generally inefficient in absorbing the heat. As a result, a substantial quantity of gas is wasted during the heating operation of the water heater 10 of the prior art.